A substantial number of aqueous streams must be treated to meet governmental laws and regulations to certify them for drinking purposes or for release into the environment. Non-limiting examples of such aqueous streams include: those emanating from municipal water supplies; those waste water streams resulting from various chemical, petrochemical, and refining processes; and, those resulting from various other industries such as the pulp and paper industry. Contaminated ground water streams must also be treated depending on their intended use. Such aqueous streams typically contain one or more impurity, such as suspended matter, organic components, etc.
One type of aqueous stream which has proven difficult to treat in a cost effective manner is an aqueous stream containing relatively low levels of organic components, typically volatile organic components. Such streams are often found in petroleum refineries and chemical plants wherein water is often initially present, or subsequently found in processes for producing various organic chemicals and products therefrom.
Various conventional techniques are presently used to remove relatively low levels of organic contaminants from aqueous streams. One leading technique involves air stripping. Air stripping techniques typically involve the use of packed, or bubble tray, columns wherein the contaminated aqueous stream is passed counter-current to a flow of air. Organic moieties pass from the aqueous liquid phase to the gaseous phase owing to the difference in concentration of organics in the two phases. That is, the organic constituents pass from the more concentrated aqueous phase to the less concentrated gaseous phase. The stripped organics are eventually released to the atmosphere with the vented air.
The use of columns has the disadvantage of being prone to fouling owing to deposits which result during the evaporation of water. Undesirable amounts of biomass also have a tendency to form in the columns. Consequently, such conventional techniques are subject to a relatively rapid decline of performance, thereby resulting in a substantial amount of down-time needed to clean the columns. Typically, two or more columns are used so that one or more columns are still in operation while one is being cleaned. The need for these additional column(s) increases the capital investment requirements of the process.
Another conventional technique consists of bubbling air directly into the contaminated aqueous stream by use of diffusers or bubblers. Although such a technique is effective for removing organics from the stream, no more than trace amounts, typically less than about 10 ppm, or even less, of the organic constituents can be present for the process to be effective. A source of compressed air is also needed, thereby increasing the capital investment and operating costs.
Other conventional techniques for removing organic constituents from aqueous streams include adsorption onto a solid adsorbent material, such as activated carbon. This technique suffers from the need to regenerate the adsorbent by removing the adsorbed organic material. This is typically done by subjecting the adsorbent to steam, which is then condensed to form a condensate stream composed of an aqueous phase and an organic phase. After gross physical separation of the organic phase from the aqueous phase, an aqueous stream is left with a relatively low, but still undesirable, level of organic constituents. These organic constituents may or may not be soluble in the aqueous stream.
Another conventional technique involves the destruction of organic constituents in the liquid phase by use of ultra-violet light; or by use of an oxidant, such as ozone, hydrogen peroxide, or both. Such techniques typically require substantial capital investment and are relatively expensive to operate and maintain.
Distillation is also an alternative technique, but is also capital intensive and costly to operate. Further, distillation is not effective for separating organic molecules which form an azeotrope with water.
Therefore, there still remains a substantial need in the art for more efficient and effective processes for removing relatively low, but still undesirable levels of organic constituents from aqueous streams.